This invention relates in general to fluid dynamic bearing assemblies. More particularly, this invention relates to disk drive motors that use fluid dynamic bearing assemblies.
In a typical magnetic disk drive, a motor rotates a magnetic disk at high speed while a transducing head uses air pressure to xe2x80x9cflyxe2x80x9d over the top surface of the disk, which is divided into a plurality of tracks. By moving the transducing head radially across the surface of the disk, the transducing head can read information from or write information to the different tracks. The faster the disk can rotate, the faster the transducing head can read and write information.
Originally, disk drive motors used standard ball bearings between a shaft and a hub. However, motors having ball bearings were known to experience problems such as vibrations that prevented information from being correctly stored and quickly accessed. This was especially true as advancements in data storage technology increased both magnetic disk storage densities and rotation speed. Fluid dynamic bearings have been proposed and used to overcome the problems associated with ball bearings. Fluid dynamic bearings are shown in U.S. Pat. No. 5,427,546 to Hensel, U.S. Pat. No. 5,516,212 to Titcomb, and U.S. Pat. No. 5,707,154 to Ichiyama.
Currently, an exemplary disk drive motor typically includes a stationary shaft on which is mounted a rotatable bearing housing. The bearing housing is secured to one or more magnetic disks. A pattern of grooves is defined on the exterior surface of the shaft or the interior surface of the bearing housing. There is no direct contact between the bearing housing and the shaft. Instead, a lubricating fluid, such as oil, is placed between the shaft and the bearing housing. Rotation of the bearing housing causes a build up of pressure in the lubricating fluid. This fluid pressure aids in forming and maintaining a fluid dynamic bearing between the shaft and the bearing housing.
Fluid dynamic bearings have advantages when applied to disk drive motors. For example, fluid dynamic bearings are generally quiet and have good run out characteristics. Also, fluid dynamic bearings occupy less space than either ball bearings or air bearings. However, fluid dynamic bearings also have disadvantages. For example, they consume more power than ball bearings or air bearings. More significantly, fluid leakage can be a problem that ultimately leads to oil xe2x80x9cstarvation,xe2x80x9d which results in increased wear and premature failure of the disk drive motor.
In an attempt to prevent leakage, the groove patterns within each fluid dynamic bearing may be designed to produce a neutral pumping action so that no fluid loss occurs in any direction.during rotation. In addition, fluid is sealed within the disk drive motor by surface-tension seals at the fluid/air interfaces and by labyrinth seals between the shaft and the bearing housing.
Yet, due to the difficulty of precisely aligning each of the grooves, it is unlikely that a fluid dynamic bearing can actually be produced in such a way that a neutral pumping is created. More significantly, surface-tension seals are very vulnerable to fluid leakage caused by centrifugal forces at high rotational speeds. Furthermore, surface-tension seals may experience blowout arising from cavitation and escaping, unwanted gases.
Unwanted gases may be entrapped in the closed space formed when the shaft is inserted into the bearing housing and, thus, affect the quality of the disk drive motor. Initial confinement of unwanted gases may be reduced by completely filling the gap between the shaft and the housing with lubricating fluid. Yet, completely filling the gap between the shaft and the housing requires filling in the vacuum, which is not cost effective.
In addition, unwanted gases can be ingested by the disk drive motor during operation, especially at high rotational speeds. The unwanted gases may expand in response to low external ambient pressure or high temperature conditions and displace the lubricating fluid within the fluid dynamic bearing, thus, adversely affecting the disk drive motor.
Venting of unwanted gases is necessary to prevent displacement of the lubricating fluid. Typically, a venting hole is provided to vent the unwanted gases through a hollow canter opening in the shaft. While placing a vent within the shaft has become a common partial-solution to the problem of gas entrapment and ingestion, fluid leakage has not been prohibited. Without a reasonable solution to the joint problems of unwanted gases and fluid leakage, one is obliged either to accept shorter useful life spans for disk drive motors or resort to large fluid reservoirs. Regardless, without a solution, the rotational speeds of disk drive motors cannot be greatly increased.
In order to solve the problems discussed above, the present invention provides a fluid dynamic bearing assembly particularly for a disk drive motor.
Accordingly, the disk drive motor includes a shaft, a thrust plate, a bearing housing positioned about the shaft, and a combination of grooves on either the shaft or the bearing housing. The grooves are, oriented to pump fluid in a specific direction when the bearing housing relatively rotates around the shaft. Vents are also positioned to allow unwanted gases to escape.